Photos of secret underground cross-border smuggling routes between Mexico and US

More than a ton of cocaine and seven tons of marijuana have been seized after authorities discovered a tunnel lasting more than half a mile under the border between the US and Mexico. The tunnel ran for about 800 metres from an elevator fitted inside a closet in a house in Tijuana to a fenced-off area in the grounds of a business selling wooden pallets in San Diego. The tunnel was only about three-feet wide, equipped with a rail system, lighting and ventilation.

20 April 2016: US attorney Laura E Duffy looks down into a hole in the ground after the discovery of a cross-border tunnel from Tijuana, Mexico to Otay Mesa, CaliforniaMike Blake/Reuters

Major drug tunnel found on San Diego/Mexico borderIBTimes UK

The discovery demonstrates the enduring appeal of tunnels to smugglers, despite the significant time and money required to build one. This is the 13th sophisticated secret passage found along California's border with Mexico since 2006, including three on the same short street in San Diego.

Dozens have been found along the US-Mexico border in recent years, mostly in California and Arizona. Many are found incomplete. The San Diego-Tijuana region is popular because its clay-like soil is relatively easy to dig into with shovels and pneumatic tools, and both sides of the border have warehouses that provide cover for trucks and heavy equipment.

23 March 2016: A US Homeland Security Investigations Special Response Team agent looks into a cross-border tunnel linking Calexico in the US and Mexicali in MexicoUS Immigration and Customs Enforcement/Reuters23 March 2016: A US Homeland Security Investigations Task Force Officer bags evidence found in a cross-border tunnelUS Immigration and Customs Enforcement/Reuters2 August 2015: A journalist walks through a tunnel discovered by the Mexican Army in TijuanaJorge Duenes/Reuters2 August 2015: A journalist walks through a tunnel that was discovered during its construction in the city of Tijuana which borders with the United StatesJorge Duenes/Reuters

In April 2015, Mexican soldiers foiled the construction of a suspected drug tunnel underneath a house near the US border, arresting nine people. Wired with lights, the tunnel was being built next to the Tijuana border crossing, south of San Diego, California, and near a Mexican air force installation, as well as a regional federal police facility. The nine people arrested were reported to work in the construction sector.

A policeman checks the entrance to a suspected drug tunnel under construction inside a wardrobe in Tijuana on April 7, 2015ReutersA makeshift cart with containers is seen at a suspected drug tunnel under construction in Tijuana on April 7, 2015Reuters

In May 2014, authorities discovered an incomplete tunnel near the US-Mexico border in Arizona. The tunnel, roughly 140ft long, was the fourth cross-border tunnel discovered in the area that year. As soon as one tunnel is shut down, the Mexican drug cartels start work on another.

22 May 2014: An underground tunnel near the US-Mexico border is filled with concrete in Arizona. The tunnel is the fourth one found this yearReuters22 May 2014: Workers fill an underground tunnel next to the US-Mexico border in Arizona with concrete. The incomplete tunnel is roughly 140 feet longReuters

In February 2014, authorities released photos of tunnels under one of the hideouts of Joaquín Guzman Loera, aka El Chapo. While not used to smuggle drugs between Mexico and the US, they did help one of Mexico's most notorious drug lords escape (if only briefly – he was caught soon afterwards). Seven homes were connected by tunnels and to the city's sewer system. The house doors were reinforced with steel, which delayed entry by law enforcement.

27 February 2014: A Mexican marine lifts a bathtub leading to a tunnel at one of the houses of Joaquin "Chapo" Guzman in CuliacanDaniel Becerril/ReutersThe bottom of a removable bathtub is pictured from inside a tunnel leading to the city's drainage system at one of the houses of Joaquin "Chapo" Guzman in Culiacan, MexicoDaniel Becerril/ReutersA journalist investigates a steel door leading from the city's drainage system into a tunnel underneath one of the houses of Joaquin "Chapo" Guzman in Culiacan, on February 25, 2014Daniel Becerril/Reuters

Guzman wasn't in jail long – he escaped via a 50cm by 50cm entrance hole in the bottom of the shower area. There was a motorcycle modified to run on rails inside the tunnel, waiting for his escape. The tunnel led to an abandoned warehouse in Almoloya de Juarez, about a mile from the high-security prison.

The shower and toilet area inside Guzman's cell in the Altiplano Federal PenitentiaryEdgard Garrido/ReutersA motorcycle modified to run on rails is seen inside the tunnelEdgard Garrido/ReutersThe abandoned warehouse in Almoloya de Juarez where the tunnel endsEdgard Garrido/Reuters

Drug-smuggling tunnels are sometimes very sophisticated. In October 2013, authorities discovered a 'super tunnel' the length of six football fields, equipped with reinforced walls, lighting, ventilation and a rail system.

Cross-border tunnels can take many months and cost hundreds of thousands, if not millions, of dollars, to construct. But the cost can be offset in just one quick shipment of marijuana, cocaine or methamphetamines. In 2014 police seized 44 tons of marijuana in Tijuana, just across the border from San Diego, California. A record haul of 150 tons of marijuana was confiscated in Mexico in 2010.

IBTimes UK looks at some of the recent tunnels that have been found linking the US and Mexico.

4 April 2014: Law enforcement officers inspect a drug tunnel found in a warehouse in Otay Mesa, San Diego. US federal agents uncovered two drug-smuggling tunnels underneath the US-Mexico border, both surfacing in San Diego-area warehouses and equipped with rail systems for moving contraband. The discovery led to the arrest of a 73-year-old woman accused of running one of the warehouses connected to a drug smuggling operationReutersA US police officer stands in a tunnel found along the Mexico/USA border at a warehouse in Otay Mesa, CaliforniaSandy Huffaker/Getty Images4 April 2014: Mexican soldiers stand guard outside a building called "Mini Warehouses of the Border" in Tijuana, where authorities discovered a cross-border drug-smuggling tunnelReuters30 October 2013: A ladder is seen at the entrance to a cross-border tunnel located underneath a warehouse in Tijuana. US authorities shut down a secret underground tunnel equipped with electricity, ventilation and a rail system for smuggling drugs between a San Diego industrial park and Tijuana. Border control and drug enforcement agents seized more than 17,000 pounds (7,700 kilograms) of marijuana and 325 pounds (147 kg) of cocaine from a rented warehouse where the passageway ended on the US sideReuters31 October 2013: A Homeland Security agent enters a tunnel used to smuggle drugs between the Otay Mesa in San Diego and TijuanaReuters31 October 2013: Seized drugs are shown in a sophisticated tunnel used to smuggle drugs between the US and Mexico. The tunnel linked an industrial park in Otay Mesa, California with Tijuana, Mexico and featured rail and ventilation systemsReuters31 October 2013: Law enforcement officials walk up to a boarded-up door of a warehouse in the Otay Mesa area of San Diego that leads to a drug smuggling tunnel. Authorities shut down a tunnel equipped with electricity, ventilation and rail system for smuggling drugs between a San Diego industrial park and Tijuana, MexicoReuters5 January 2013: A ventilated tunnel is seen in a cellar in Mexicali, MexicoReuters6 December 2012: Journalists climb down a ladder in a cross-border tunnel located underneath a water purifying plant in Tecate, Mexico. The tunnel, which was still under construction, was found due to an anonymous tip. Seven people who were found working on the tunnel when it was discovered were arrested by the authoritiesReuters12 July 2012: A military truck is parked outside a recycling plant where soldiers found a tunnel under construction at the Otay Mesa industrial park in TijuanaReuters12 July 2012: Soldiers inspect the entrance of a cross-border tunnel at a warehouse in Tijuana. The Mexican army located a tunnel measuring 383 yards (350 metres) running to California from Mexico and seized some 50 tons of marijuana. The tunnel was still under construction but had ventilation and lightingReuters13 July 2012: Thirty-nine pounds of methamphetamines that were smuggled through a cross-border tunnel are pictured in this photo taken by the DEA in San Luis, ArizonaReuters30 November 2011: Agents prepare to enter a tunnel found in a warehouse in Otay Mesa, California, leading to a small business building in Tijuana. The tunnel was kitted out with an elevator, wood flooring and lightingSandy Huffaker/Getty Images29 November 2011: Packages containing marijuana are seen at the Mexican entrance to a tunnel running between Tijuana and San DiegoReuters30 November 2011: A soldier looks down into a tunnel running under the US border from Tijuana. The tunnel was equipped with a hydraulic entry, an elevator and electric rail carsReuters29 November 2011: Soldiers stand guard outside a warehouse where a cross-border tunnel was found in Tijuana. The tunnel measured around 800 metres and ran from the Mexican border city of Tijuana to Otay Mesa industrial park south of San DiegoReuters30 November 2011: An official investigates a sophisticated tunnel found in a warehouse in Otay Mesa, CaliforniaSandy Huffaker/Getty Images30 November 2011: A soldier stands guard next to packages containing marijuana found in a tunnel in Tijuana . Authorities seized more than 32 tons of marijuana. The tunnel was equipped with a hydraulic entry, an elevator and electric rail carsReuters16 November 2011: A journalist walks through a tunnel in Tijuana . Police discovered a tunnel running to California from Mexico, and seized 14 tons of marijuanaReuters16 November 2011: Packages containing marijuana with stickers showing Captain America, a Sprite can and a bottle of Bud Light are seen in Tijuana . Police discovered a "major cross-border drug tunnel" linking warehouses in an industrial park south of San Diego and the Mexican border city of TijuanaReuters25 November 2010: A soldier sits on a cart inside a "narco-tunnel" found in Tijuana and crossing into the US.Reuters26 November 2010: A Mexican soldier patrols a 700m tunnel discovered at a warehouse in Tijuana, Baja California state, Mexico, leading to San DiegoFrancisco Vega/AFP25 November 2010: A cache of marijuana is seen after it was recovered from a tractor trailer truck found near a warehouse in San Diego. US border agents found a half-mile-long tunnel under the US-Mexico border and seized a significant amount of marijuana at the San Diego area warehouse where it endedReuters3 November 2010: An agent from the US Immigration and Customs Enforcement (ICE) stands guard in front of an industrial building near the Mexican border in Otay Mesa, San Diego. Police found a sophisticated smuggler's tunnel the length of six football fields linking Southern California with MexicoReuters3 November 2010: A Mexican soldier flashes his torchlight into a tunnel in Tijuana. Agents recovered more than 25 tonnes of marijuana in seizures related to the investigation in both California and Mexico and arrested a US citizen and his Mexican wifeReuters4 November 2010: A homeland security agent crawls through a tunnel connecting a warehouse in Otay Mesa, California, to a warehouse in Tijuana, MexicoSandy Huffaker/Getty Images4 November 2010: The view out of a drug tunnel found by agents in a warehouse near the US-Mexico borde. Authorities confiscated over 30 tons of marijuana in the tunnel, which connected to a warehouse in TijuanaSandy Huffaker/Getty Images4 December 2007: An image of the Virgin of Guadalupe is seen in a tunnel, nearly a mile long, running into California from Tecate near the Pacific coast. Gunmen killed the police chief of Tecate, a Mexican city bordering California, by shooting him some 50 times in an apparent revenge attack after police found the drug-smuggling tunnel under the borderReuters30 January 2006: US immigration and customs enforcement officers stand guard over an entrance to a drug tunnel found in a warehouse in Otay Mesa, California. The tunnel was 2,400 feet long and furnished with lighting, ventilation and equipment to pump out ground waterSandy Huffaker/Getty Images